Displacement sensors which come in many different forms are usually designed to give a quantitative measurement of the displacement being measured. Often, however, a quantitative output is not required, and an indication that movement has occurred suffices.
One particular type of displacement sensor with which this invention is concerned is respiratory monitors. A respiratory monitor is already known which has a sensor for detecting mechanically the breathing of a patient and an electronic control unit which provides output signals determined by the state of the sensor. The sensor in the known monitor comprises a flattened, hollow, rubber or plastic bulb containing an air bubble and connected by tubing to an air-pressure transducer. The sensor is strapped around a patient's chest or abdomen whereby expansion of the lungs causes the bulb to be squashed and the air pressure therewithin to be increased. The pressure increase results in an electrical signal from the transducer which is detected by the electronic control unit to provide an indication of the incidence of respiration by the patient.
A problem in practice with this known system is that it reacts only to increases in pressure at the transducer. Thus, for example, if the patient should roll over and lie on top of the sensor it could be deformed permanently so that no further increase in pressure can result even if the patient is, in fact, breathing normally. Thus, the known system is liable to false alarms due primarily to the type of sensor used but also to the fact that the electronic control unit only reacts to an increase in pressure in the sensor.